1. Field of the Invention
Embodiments of the invention relate to semiconductor devices in which separation of an electrode is prevented.
2. Related Art
Among semiconductor devices heretofore used as power devices, some semiconductor devices using silicon as their semiconductor materials are the mainstream. Silicon carbide (SiC) that is a wide-gap semiconductor has physical properties including thermal conductivity three times as high, maximum electric field intensity ten times as high and electron drift velocity twice as high as silicon. For this reason, in recent years, applications of SiC as power devices which can operate with high breakdown voltage and low loss at a high temperature have been researched.
As for the structure of a power device, a vertical semiconductor device with a back-side electrode provided with a low-resistance ohmic electrode on the back side is the mainstream. Various materials and structures have been used for the back-side electrode. As one of those, there has been proposed a laminate of a titanium layer, a nickel layer and a silver layer (see, for example, Japanese patent application no. JP-A-2007-184571 (also referred to herein as “PTL 1”)), a laminate of a titanium layer, a nickel layer and a gold layer (see, for example, Japanese patent application no. JP-A-2010-86999 (also referred to herein as “PTL 2”)), or the like.
In a vertical semiconductor device using SiC as typified by a Schottky barrier diode, used is a method in which a nickel silicide layer is formed by heating after an SiC substrate is filmed with a nickel layer, and an ohmic contact is formed between the SiC substrate and the nickel silicide layer (for example, see the following PTL 1 and the following PTL 2). However, there is a problem that, when a back-side electrode is formed on the nickel silicide layer, the back-side electrode is separated from the nickel silicide layer easily.
Therefore, there has been proposed a technique of a back-side electrode in which a titanium layer, a nickel layer and a silver layer are laminated in this order after a nickel layer remaining on the surface of a nickel silicide layer when the nickel silicide layer is formed is eliminated from the surface of the nickel silicide layer (see, for example, Japanese patent application no. JP-A-2008-53291 (also referred to herein as “PTL 3”)). It has been suggested that a portion of a cathode electrode in contact with the nickel silicide layer is made of another metal than nickel so as to suppress a defect of separation. It has been also suggested that even if a layer where carbon has been precipitated is formed between the nickel silicide or the like and the cathode electrode, the layer where carbon has been precipitated can be removed together with the nickel layer so that separation can be prevented.
In addition, there has been proposed a technique in which carbide formed in the surface of a nickel silicide layer is removed to improve the adhesion of a back-side electrode (see, for example, Japanese patent application no. JP-A-2003-243323 (also referred to herein as “PTL 4”)).
In PTL 3 or PTL 4 belonging to the background art, there is a problem that the adhesion between a nickel silicide layer and a titanium layer of a cathode electrode layer is low even in a back-side electrode with a configuration conceived to be capable of suppressing a defect. For example, there is a problem that the back-side electrode may be separated from the nickel silicide layer when a semiconductor device is diced.
For example, in a method for manufacturing a back-side electrode for an SiC semiconductor device as described in PTL 3, a nickel layer is formed on an SiC substrate, a nickel silicide layer is formed by subsequent heating, and an ohmic contact is formed between the SiC and the nickel silicide layer.
According to the description of PTL 1, nickel silicide is produced by solid-phase reaction expressed by the following reaction formula.Ni+2SiC→NiSi2+2C
Carbon (C) produced in the aforementioned reaction formula exists as an unstable supersaturation state or a micro-precipitate so as to be dispersed all over the inside of the nickel silicide layer. Once heating treatment is performed after silicide is formed, the C is discharged at once, and aggregated (precipitated) stratiformly as a precipitate regarded as graphite on and inside the silicide layer. The precipitate is a material which is fragile and poor in adherability. Accordingly, the precipitate is broken easily when slight stress acts thereon. Thus, a back-side electrode metal layer formed on the silicide layer is separated.
As described above, in the process for manufacturing an SiC semiconductor device, after Ni for forming an ohmic electrode is deposited on an SiC substrate, the SiC substrate and the Ni of the electrode react with each other due to heating treatment so that nickel silicide is formed. Further, due to various heating treatments carried out in a step of forming a Schottky electrode of the semiconductor device, and so on, there is a problem that carbon in the SiC substrate is diffused and precipitated in the nickel silicide or on the surface of the nickel silicide.